Houses with the newer insulated windows are going to be susceptible to seal failure. This will become more noticeable when you start having greater temperature differences between the inside and outside of the house. The visual evidence will mostly be milky white staining or water drips, drips, or fogginess on the glass panes in windows or doors that cannot be cleaned off. The reason for this is that the staining or moisture is actually trapped between the inner and outer panes of glass.
To better understand why this is, let me explain. In these newer windows and doors, the glass panels are actually a laminate of two pieces of glass (inner and outer panes) that are held together and separated by a gasketed band around the perimeter of the glass panel. The resultant air space in between the glass is filled with a dry, inert gas such as argon. The total effect here is a virtually fog-free window with a small amount of insulative value (understand that the insulation is in the trapped air-space, not in the two pieces of glass, which has no insulative value), that is required by residential and commercial building codes in many areas as a mandatory energy conservation measure.
The fogging and/or staining appears when the gasket around the perimeter fails, thereby allowing moisture laden air to replace the dry argon. Once you have a sixeable temperature difference, such as hot inside and cold outside (typical winter) or hot outside and cold inside (typical summer). The tricky thing with these systems is that you may have two windows with failed seals, each on opposite sides of the house, and only one may show signs of failure in the morning. But, in the afternoon, they switch. Also, just because you have one failed seal, does not mean they are all bad, and they don't all just fail at the same time. This is why a home inspector will usually include language in the inspection report to the effect of: "while only "x" number of windows were noted to have failed seal, there may be more present which were not readily apparent."
Questions about failed insulated glass seals? Leave them below in the comments and I'll get back to you.